1. Field of the Invention
The present invention relates to a distance measuring device that measures the distance to an object, and more particularly to an active type distance measuring device suitably used for cameras or the like.
2. Related Background Art
Conventionally, as an active type distance measuring device used for cameras or the like, the following distance measuring device is known. That is, beam of light is projected from an infrared light emitting diode (hereinafter, referred to as “IRED”) to an object to be measured, the reflected light of the projected beam of light is detected by a position sensitive detector (hereinafter, referred to as “PSD”), and the signal output from the PSD is calculated and processed by signal processing circuit and an arithmetic circuit and output therefrom as distance information; thereby the distance to the object to be measured is calculated by a CPU. Further, in the case where the distance measurement is made by only one light projection, an error may be generated. Accordingly, it is preferred that a distance measuring routine including light projection with light projecting means, light detection with light detecting means, output of output signal with light detecting means, and discharge or charge of an integration capacitor is carried out several times to obtain plural distance information, and the plural distance information are integrated by an integration circuit at predetermined intervals and averaged. The integration of the distance information with the integration circuit is made by discharging the integration capacitor; and from that state, a voltage corresponding to the distance information is applied to accumulate electric charge.
According to the distance measuring device as described above, the number of times of repetition of the distance measuring routine (hereinafter, referred to as “number of times of distance measuring routine”) is uniformly set up in the following manner. That is, the number of times of the distance measuring routine is uniformly determined irrespective of the difference in the integration current and the capacity of integration capacitor depending on the individual product; and assuming an individual product of which capacity of the integration capacitor is the smallest and the integrated current is the largest in the difference range, and in such individual product, even when the amount of the accumulated charge is the largest (object to be measured is close), the amount of the accumulated charge does not exceed the capacity of the integration capacitor.
However, in the above distance measuring device, since the number of times of the distance measuring routine is uniformly determined, there may be a case that, when the capacity of the integration capacitor is large or the integrated current is small due to the difference of the product, the integration operation, in which the capacity of the integration capacitor is fully used, can not be made. Thus, satisfactory measuring accuracy is not obtained.
FIG. 8 is a timing chart of conventional distance measuring device in the case where the distance measurement is made at the closest distance capable of photographing for a camera. The abscissa axis represents integration time; and the ordinate axis represents voltage of the integration capacitor. Conventionally, the number of times of the distance measuring routine is fixed irrespective of the individual product. Therefore, as shown in FIG. 8, in an individual product (distance measuring device 1) of which capacity of the integration capacitor is the smallest and the integration current is the largest, electric charge is accumulated until the voltage of the integration capacitor reaches the substantially maximum value; thus the range of available AD signal value can be used effectively. However, in the case of many average individual products (distance measuring device 2), since the integration capacitor is not charged up to the maximum voltage, satisfactory resolution of the AD signal cannot be obtained; thus, accuracy of the distance measurement cannot be obtained. Further, in the individual product (distance measuring device 3), of which capacity of the integration capacitor is the largest and the integration current is the smallest, the utilization rate of the capacity of the integration capacitor and the range of the available AD signal value become lowest.
In order to solve the above problems, a distance measuring device disclosed in Japanese Unexamined Patent Application Publication (Tokukai) No. H5-280973 (Patent Document 1), it is arranged so that elements, which change the dynamic range such as the number of times of the distance measuring routine, integration time and the capacity of the integration capacitor, are controlled.
Patent Document 1: Japanese Unexamined Patent Application Publication (Tokukai) No. H5-280973
However, in the above distance measuring device, since it is arranged so that elements are changed every time of the distance measurement, the operation to calculate the distance becomes complicated and the measuring time becomes long. When the distance measuring time of the distance measuring device is long, in the case that the distance measuring device is used in a camera or the like, there are such problems that an appropriate shutter chance can not be obtained or the like.